The present invention relates to gratings written in optical fibers, such as Bragg gratings, and more specifically it relates to the polarization mode dispersion of such gratings.
Writing gratings in fibers is based on inducing index variations in the fiber by exposure to ultraviolet light. Exposing an appropriately doped optical fiber to ultraviolet light creates areas at more or less regular intervals in which the index varies. An optical component whose index varies along the fiber is called a written grating.
In the simplest case of a monomode fiber having an asymmetrical section conveying two modes that are linearly polarized in orthogonal directions, the birefringence of the fiber is defined as the index difference between two main polarization axes. The polarization direction in which the index is higher is called the linear birefringence axis. D. N. Payne et al., in xe2x80x9cDevelopment of Low- and High-birefringence Optical Fiberxe2x80x9d, IEEE Journal of Quantum Electronics, vol. QE-18 no. 4 (1982), pages 477-487, provide a definition of birefringence and discuss the various parameters that affect the birefringence of an optical fiber. Curvature, pressure, rotations of the fiber, magnetic fields and temperature are stated to influence birefringence.
Polarization mode dispersion is representative of the dispersion of a fiber or a component written in a fiber as a function of the polarization axes of the fiber; it is defined as the amplitude of the variation in the group delay time when the polarization of the incident light is varied over all possible polarization states. This quantity is directly linked to the axes of polarization in the fiber. There is a relationship between birefringence and polarization mode dispersion, at least in the case of linear birefringence. The relationship in the case of a fiber or in the case of a grating written in a fiber is given by I. Riant et al. in xe2x80x9cPolarization mode dispersion analysis in fiber chromatic dispersion compensatorsxe2x80x9d, OFC""99, TuS2-1 p269. The remainder of the present description uses a formulation in terms of birefringence or in terms of polarization mode dispersion.
The birefringence or polarization mode dispersion of a written grating is a problem in that it makes the properties of the grating (in particular its reflectivity) polarization-dependent. This problem is particularly serious for transmission applications at high bit rates (in excess of 10 Gbit/s per channel).
A. M. Vengsarkar et al., in xe2x80x9cBirefringence reduction in side-written photoinduced fiber devices by a dual exposure methodxe2x80x9d, Optics Letters, vol. 19 no. 16 (1994), pages 1260-1262, raise the problem of the increased birefringence when side-writing gratings using ultraviolet light. The paper explains that the asymmetry of side-writing is one cause of increased birefringence. To reduce birefringence it is proposed to use a double-exposure technique or to expose a cylindrical fiber from both sides simultaneously.
T. Erdogan and V. Mizrahi, in xe2x80x9cCharacterisation of UV-induced birefringence in photosensitive Ge-doped silica optical fibersxe2x80x9d, J. Opt. Soc. Am. B, vol. 11 no. 10 (1994), pages 2100-2105, propose an explanation of the increased birefringence in gratings written in a fiber. They show that the use of ultraviolet light polarized along the propagation axis of the fiber to write the grating reduces birefringence. This solution has the drawback of reducing the photosensitivity, and therefore the maximum modulation of the index, of the grating.
The above-mentioned paper by I. Riant et al. discusses the influence on the polarization mode dispersion of a photo-induced grating of the polarization mode dispersion of the original fiber. It suggests writing gratings in low-birefringence fiber to limit the polarization mode dispersion of the gratings obtained. However, this solution imposes a limit in terms of the fibers in which a grating can be written.
The invention addresses the problem of birefringence in gratings written in an optical fiber. It proposes a simple solution that applies independently of the type of fiber used to write the grating. It enables writing by the various methods known in the art, with or without double exposure.
To be more precise, the invention proposes an optical fiber incorporating a written grating, the optical fiber having a curvature in a plane substantially perpendicular to the axis of the birefringence induced by writing the grating.
In a preferred embodiment the angle between said plane and the axis of the birefringence induced by writing the grating is in the range from 65xc2x0 and 115xc2x0.
The curvature preferably extends at least into the area in which the grating is written.
In one embodiment the curvature is constant and has a value in the range from 5 mxe2x88x921 to 10 mxe2x88x921.
The written grating is advantageously a Bragg grating.
The invention also relates to an optical component including an optical fiber incorporating a written grating, wherein said fiber has a curvature in a plane substantially perpendicular to the axis of the birefringence induced by writing the grating.
The invention also proposes a chromatic dispersion compensator including at least one fiber as defined hereinabove.